This invention relates in general to amplifiers and more particularly to amplifier circuits having means for minimizing the input-capacitance thereof. Still more particularly, this invention relates to amplifier circuitry useful in driving the guard shield of a highly sensitive proximity sensor which operates by detection of varying capacitance at a remotely disposed sense electrode.
Proximity sensors are well known in the prior art. Such systems often operate by utilizing magnetic or optical detectors which generate a signal in response to the presence or absence of a object. More recently, capacitive proximity sensors have been utilized which use a high impedance AC signal to detect variations in the capacitive loading of the AC signal due to the presence of an object. However, these capacitive proximity sensors suffer from a severe shortcoming in that small variations in parasitic capacitance in the associated electronic circuitry may cause faulty indications. Typically, these systems are sensitive to capacitive variations at the sense electrode on the order of 0.005 picoFarads. Thus, those skilled in the art will appreciate that parasitic capacitance can generate a major problem when present in the associated circuitry utilized with a proximity sensor of this type.
A second shortcoming of these capacitive proximity sensors is also related to the highly susceptible nature of the high impedance AC signal utilized. It is often necessary to dispose the electronic circuitry associated with these sensors at some distance from the sense electrode itself due to an adverse or hostile environment in which an object must be sensed. Examples of this situation include the various hostile environments to which a semiconductor wafer must be exposed during fabrication. The necessity of utilizing remote sense electrodes generates a second problem with the stray capacitance found in most electrical transmission lines. The approach typically utilized in the prior art to solve this problem is to drive an outer conductor or "guard shield" with an buffered version of the signal being guarded. This maintains both the inner conductor and the guard shield at identical electrical potentials and minimizes the capacitance in the transmission line. However, the necessity of amplification introduces additional electronic circuitry which necessarily includes additional parasitic capacitance.
One approach utilized in the prior art to offset the effect of parasitic capacitance is to capacitively inject current onto the signal node being guarded equal and opposite to that being lost to parasitic capacitance. One major disadvantage to this approach is the difficulty in making such neutralization independent of temperature. Parasitic capacitances in semiconductor devices are highly subject to temperature variation and may fluctuate greatly in response to these temperature variations. Thus, it is very difficult to choose a particular level of compensation for parasitic capacitance which will be effective throughout a large temperature range.